This invention relates to an improvement of the speed control system of an induction motor for driving a vehicle, for example, an electric motor car, or other loads requiring various speed-torque characteristics.
When compared with a direct current motor, since a cage type induction motor is advantageous in that is has small size, light weight, is not expensive, and is rugged and free from maintenance troubles, a variable speed drive system for an electric motor car which employs an induction motor combined with a controllable inverter has recently been developed.
Speed control systems for an induction motor are classified into two types, one the constant V/f control system (speed control system for an induction motor utilizing a so-called variable voltage, variable frequency inverter), and the other the constant fs control system wherein v represents the line voltage, f the line frequency and fs the slip frequency of the induction motor.
According to the constant V/f control system, as diagrammatically shown in FIG. 1, a rectifier 12 and an inverter 13 are connected in series between a source of alternating current 10 and an induction motor 11. Although any one of the many well known rectifiers and inverters may be used, such as a grid controlled mercury arc discharge device, with the recent development of power semiconductors such as the thyristor and power transistor it is advantageous to compose the rectifier 12 of bridge connected diodes or thyristor, and the inverter 13 with bridge connected thyristors. Since thyristors having a rating of 1300-2500V and 300-800 amperes are readily available on the market, a thyristor inverter is preferred for loads having relatively small value, for example an induction motor for driving a motor car. The control signal for the rectifier is generated by a circuit including a frequency setter 14, a frequency-voltage converter 15 which generates a voltage signal proportional to the input frequency, a current limiter 16 and a control voltage generator 17 which generates a gate pulse for the rectifier in response to the voltage signal. The frequency setter 14 is connected to the inverter 13 through a circuit including an oscillator 18 which varies its oscillation frequency in accordance with the setting of the frequency setter 14 and a frequency controller 19 which generates a gate pulse having a frequency determined by the output from the oscillator 18. As shown, the current limiter 16 is connected to the output of a current transformer CT connected on the primary side of the induction motor 11 for the purpose of limiting the primary current to a safe value.
As is recognized in the art, this control system provides a constant V/f control (constant magnetic flux) over a wide range of speed variation in which as the motor load varies the slip frequency is automatically varied in accordance with the characteristic of the motor, thus operating the motor under the steady state.
However, it is difficult to maintain the efficiency and the power factor of the motor always at high values because the slip frequency varies greatly with the load. Moreover, the starting characteristic of the motor is not good and stalling often occurs during transient conditions.
At the time of the regeneration operation (power regeneration or braking) the operating frequency of the oscillator 18 is decreased below the frequency corresponding to the synchronous speed of the motor to render the slip frequency fs negative. However, under such conditions, the slip frequency is not definite so that it is impossible to respond to a rapid load variation thus resulting in a hunting. As a result, it is impossible to provide a stable and continuous control.
According to the constant slip frequency (fs) control system it is possible to provide a DC series motor characteristic for an induction motor by controlling the slip frequency thereof to maintain it at a constant value. It is also possible to improve the efficiency and power factor of the motor over the constant V/f control system provided that the core of the motor is not saturated.
As shown in FIG. 2, a typical constant slip frequency control system comprises a rectifier 22 and an inverter 23 which are connected in series between a source of alternating current 20 and an induction motor 21. The output of a speed reference setter 24 is applied to the rectifier 22 through a circuit including a comparator 24c, a current limiter 25 and a control voltage generator 26. On the other hand, the output of a tachometer generator TG coupled to the induction motor 21 is applied to the inverter 23 through a circuit including a level adjuster 27 which converts the output of the tachometer generator TG into a frequency signal fn having a synchronous speed equal to the rotating speed n of the motor, an adder 28 which adds the output of the level adjuster 27 to the output of a slip frequency setter 28s and a frequency controller 29 which converts the output of the adder into a gate pulse for controlling the inverter. As before, the current limiter 25 is set in accordance with the output of a current transformer CT connected on the primary side of the motor 21 and the output of the tachometer generator TG is also applied to comparator 24c.
According to this system, the frequency signal fn which utilizes the speed signal generated by the tachometer generator TG as the synchronizing signal is added to a definite slip frequency signal fs, and a closed loop is formed with respect to frequency. The speed of the motor is varied by varying the output voltage V of rectifier 22.
Although this system is advantageous in that it can improve the starting characteristic, can eliminate the unstable phenomenon (stalling) during transients, and can improve the efficiency and power factor of the motor at a specific load corresponding to a selected slip frequency, it is inevitable that there will be a decrease in the power factor and efficiency for loads larger or smaller than the specific load according to the saturation of the motor.
When the constant slip frequency control is employed, the motor exhibits a characteristic similar to that of a direct current series motor but it is difficult to stably and continuously control the regeneration torque over the entire speed range for the same reason that the regenerative braking of a conventional DC series motor is difficult.